The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Recently, increase of oil price has drawn the attention of the auto industry to develop a vehicle having high fuel efficiency.
As one approach, research for an automatic transmission has been made to provide better drivability and less fuel consumption by achieving more speed stages.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components (particularly, planetary gear sets) increase, and as a result, a lengh of the transmission increases. Therefore, mountability, production cost, weight and power delivery efficiency may be deteriorated, in particular, in a case of a front wheel drive vehicle where an engine is laterally mounted.
Accordingly, development of a planetary gear train that improves efficiency with a small number of components may be provided in order to increase a fuel economy enhancement effect through the multiple speed stages.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, a conventional 8-speed automatic transmission typically includes three to four planetary gear sets and five to six control elements (frictional elements). In this case, since the length of the automatic transmission increases, mountability in a front wheel drive vehicle may be deteriorated.
Recently, one planetary gear set is disposed above another planetary gear set, but structures of automatic transmissions to which parallel planetary gear sets is applied are very limited.
In another way, dog clutches instead of control elements of wet-type are used. However, shift feel can be undermined.
In addition, since a conventional 8-speed automatic transmission has gear ratio span of 6.5-7.5, we have discovered that improvement of power performance and fuel economy may be minimal.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.